Cleaning up Older Mac.

There is no magic wand app. In fact, most third party cleaning and optimizing app's will make a bigger mess and possible even render your iMac useless.

For starters,

1. reduce the number of items set to startup at login.
2. remove unwanted/unused third party app's.
3. thin down the number of dock icons.
4. empty out your downloads folder.
5. clean up your desktop icons.
6. use Safe Mode, then restart.

Then use EtreCheck Pro and post back here with that Report.

see > https://etrecheck.com

for help posting your EtreCheck Pro Report here for evaluation,

see > https://discussions.apple.com/docs/DOC-250000211